The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.

Organic bulk heterojunction (BHJ) solar cells comprising conjugated polymers (as electron donor) and fullerene derivatives (as electron acceptor) deposited from solution make low cost photovoltaic-energy conversion feasible. [ 1 , 2 ] The active layer of BHJ solar cells comprises an interpenetrating network of polymer and fullerene domains that forms during deposition and drying. The energy-conversion process involves the creation of excitons upon light absorption which are dissociated into free charges at the donor/acceptor interface and are further transported towards the respective electrodes. Thus, for a given combination of donor and acceptor materials, the effi ciency of a BHJ critically depends on its nanoscale structural properties. Ideally, BHJs must have a large interface with domain sizes comparable to the exciton diffusion length (a few nano meters) and effective percolation paths to the electrodes, with balanced transport of holes and electrons. In reality, because of the variety and complex interplay of thermodynamic and kinetic factors that infl uence the resulting nanomorphology, optimization of the microstructure and the morphology is a laborious task that involves trialling many processing protocols. [3][4][5] Over recent years numerous studies have been performed on the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) with the aim of advancing the understanding of the structure-property-performance relationships. The development of P3HT crystalline order with a π -π stacking of the molecules has been found to be an important factor in ensuring suffi cient hole-carrier mobility for charge extraction. [6][7][8] Other structural issues of importance are the relative orientation of P3HT crystallites (due to their anisotropic electrical charge mobility) and the distribution of amorphous P3HT regions (which are expected to affect the electronic interconnection among P3HT domains). [ 6 , 9 ] However, the relative importance of the many implied microstructural features on solar-cell performance and the factors determining the development of specifi c structures during blend solidifi cation are still poorly understood. As many results of studies into the effect of processing conditions on nanomorphology cannot be transferred to an industrially relevant process (because the coating shear forces and solvent-evaporation conditions are different), we have chosen doctor-blading as a lab-scale fabrication method which is scalable to roll-to-roll (R2R) processing. [ 10 , 11 ] Here, the effect of substrate temperature on the structural evolution of P3HT:PCBM blends has been investigated using in situ grazing incidence X-ray scattering (GIXS). In situ X-ray measurements allow real-time observation of the emergence and evolution of the blend microstructure during solvent evaporation. The characterization of the dried fi lms is complemented with ex situ X-ray measurements, optical-absorption data, and the application of atomic force microscopy (AFM) to explore the de...

A three-pronged approach has been used to design rational improvements in self-assembled monolayer field-effect transistors: classical molecular dynamics (MD) simulations to investigate atomistic structure, large-scale quantum mechanical (QM) calculations for electronic properties, and device fabrication and characterization as the ultimate goal. The MD simulations reveal the effect of using two-component monolayers to achieve intact dielectric insulating layers and a well-defined semiconductor channel. The QM calculations identify improved conduction paths in the monolayers that consist of an optimum mixing ratio of the components. These results have been used both to confirm the predictions of the calculations and to optimize real devices. Monolayers were characterized with X-ray reflectivity measurements and by electronic characterization of complete devices.

Swelling of Hummers graphene oxide (HGO) membranes in a set of progressively longer liquid alcohols (methanol to 1-nonanol) was studied using synchrotron radiation XRD after air ageing over prolonged periods of time.

We report an in situ X-ray investigation of the composition dependence on the structural evolution during drying of doctor-bladed blends of poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). This study enables an observation of the microstructure evolution in real time during blend crystallization. P3HT:PCBM blends with ratios of 1:0.5, 1:0.8, and 1:2 exhibit differing structural evolution during the course of solvent evaporation resulting in a different microstructure of the blends upon solidification. Large excess of PCBM over the eutectic composition impedes the π−π packing of P3HT chains and leads to a not yet observed diffraction feature with an associated spacing of 12.6 Å, which might originate from a disordered phase of intimate mixed P3HT and PCBM molecules. This work provides a microscopic understanding of the composition dependence of the film formation from solution. The structural results are discussed in relation to the composition dependence of photovoltaic performance previously reported.

Neutron reflectometry was used to study the assembly of magnetite nanoparticles in a water-based ferrofluid close to a silicon surface. Under three conditions, static, under shear and with a magnetic field, the depth profile is extracted. The particles have an average diameter of 11 nm and a volume density of 5% in a D2O-H2O mixture. They are surrounded by a 4 nm thick bilayer of carboxylic acid for steric repulsion. The reflectivity data were fitted to a model using a least square routine based on the Parratt formalism. From the scattering length density depth profiles the following behavior is concluded: the fits indicate that excess carboxylic acid covers the silicon surface and almost eliminates the water in the densely packed wetting layer that forms close to the silicon surface. Under constant shear the wetting layer persists but a depletion layer forms between the wetting layer and the moving ferrofluid. Once the flow is stopped, the wetting layer becomes more pronounced with dense packing and is accompanied by a looser packed second layer. In the case of an applied magnetic field the prolate particles experience a torque and align with their long axes along the silicon surface which leads to a higher particle density.

Mixed films of perfluorinated pentacene and diindenoperylene are studied for their structural and optical properties on SiO 2 and glass substrates. Grazing incidence Xray diffraction shows that the compounds mix on the molecular level. Two structures are identified. Their out-ofplane lattice spacing is determined by X-ray reflectivity, and their growth morphology is characterized using atomic force microscopy. It is found that one structure consists of molecules in a standing-up and the other in a lying-down orientation. The uniaxial dielectric function of mixed films is determined with spectroscopic ellipsometry, and the in-plane optical extinction coefficient is measured by real-time differential reflectance spectroscopy during growth. The question of intermolecular coupling between perfluorinated pentacene and diindenoperylene is discussed.

The packing of anisotropic ultranarrow nanoparticles ( r